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WebAssembly · 23 Breaking Down Cold Execution
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Stage: Correctness and Bottleneck Evaluation · External example · preview instructions · Storage ID: webassembly-step-23
Goals and learning outcomes
- Learning outcome: explain the role of Cold costs. Input cases: the first request in a new process.
- Deliverable: per-stage durations and a table of included work. Explain the rationale for handling Measurement boundaries, linking it to the code.
Prerequisites
Prepare the experiment results for Cache and Version from Unit 22, “Module Caching and Versions”.
Key concepts and explanations
Cold costs
File loading·compilation·instantiation·the first call are distinct stages. A user's first request may encounter them together.
Measurement boundaries
The existing coldHarnessTotalMs includes validation and other experiments. Do not mistake it for pure initialization time.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The iframe preview is an experiment with WebAssembly.Memory and JS reference computations; do not present it as execution of an actual Wasm module. Practice with actual modules in a separate Node 22+ environment or a controlled local web project. Use kernel.wat·kernel.wasm·benchmark.mjs in the existing src/labs/webassembly-02·03 folders, and rebuild with the official wat2wasm tool when modifying WAT. Preserve CSP·isolation, and record measurements yourself.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In day-23.mjs in a separate practice project, map the benchmark's timer boundaries into a table and write a new measurement wrapper. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: the first request in a new process. Baseline: kernel only. Run under the baseline conditions, then record observations and errors in notes/day-23.md.
- Variable: measurement scope; changed value: entire first request. Run again through the same input path. Deliverable to save: per-stage durations and a table of included work. Hypothesis to check: The entire first request adds preparation·copy costs. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| measurement scope | kernel only | entire first request | The entire first request adds preparation·copy costs. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: per-stage durations and a table of included work. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: Cold costs and Measurement boundaries. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Cold costs affect in this exercise? Input cases: the first request in a new process.
- Variable: measurement scope. Hypothesis to check: The entire first request adds preparation·copy costs. Did the actual results agree with the hypothesis? Explain the evidence in terms of Measurement boundaries.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Standalone WAT + Node assertions; iframe CSP blocks Wasm compilation.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Module Caching and Versions · Next: Warm Repeats and Warm-up